Let's suppose we have a line like this:
aaa authentication login default group radius group tacacs+ local
According to the IOS 12.2 documentation:
If R1 authenticates the user, it issues a PASS response to the network access server and the user is allowed to access the network. If R1 returns a FAIL response, the user is denied access and the session is terminated. If R1 does not respond, then the network access server processes that as an ERROR and queries R2 for authentication information
It is important to remember that a FAIL response is significantly different from an ERROR. A FAIL means that the user has not met the criteria contained in the applicable authentication database to be successfully authenticated. Authentication ends with a FAIL response. An ERROR means that the security server has not responded to an authentication query. Because of this, no authentication has been attempted. Only when an ERROR is detected will AAA select the next authentication method defined in the authentication method list.
My question is: How does a Cisco device exactly perform the check on an authentication server? Is it possible to modify/extend this method?
The motivation behind this question is to ensure that the reachability creteria meet some specific L7 methods rather than L3/L4.